Documentation/leds/ledtrig-transient.txt

   1 LED Transient Trigger
   2 =====================
   3
   4 The leds timer trigger does not currently have an interface to activate
   5 a one shot timer. The current support allows for setting two timers, one for
   6 specifying how long a state to be on, and the second for how long the state
   7 to be off. The delay_on value specifies the time period an LED should stay
   8 in on state, followed by a delay_off value that specifies how long the LED
   9 should stay in off state. The on and off cycle repeats until the trigger
  10 gets deactivated. There is no provision for one time activation to implement
  11 features that require an on or off state to be held just once and then stay in
  12 the original state forever.
  13
  14 Without one shot timer interface, user space can still use timer trigger to
  15 set a timer to hold a state, however when user space application crashes or
  16 goes away without deactivating the timer, the hardware will be left in that
  17 state permanently.
  18
  19 As a specific example of this use-case, let's look at vibrate feature on
  20 phones. Vibrate function on phones is implemented using PWM pins on SoC or
  21 PMIC. There is a need to activate one shot timer to control the vibrate
  22 feature, to prevent user space crashes leaving the phone in vibrate mode
  23 permanently causing the battery to drain.
  24
  25 Transient trigger addresses the need for one shot timer activation. The
  26 transient trigger can be enabled and disabled just like the other leds
  27 triggers.
  28
  29 When an led class device driver registers itself, it can specify all leds
  30 triggers it supports and a default trigger. During registration, activation
  31 routine for the default trigger gets called. During registration of an led
  32 class device, the LED state does not change.
  33
  34 When the driver unregisters, deactivation routine for the currently active
  35 trigger will be called, and LED state is changed to LED_OFF.
  36
  37 Driver suspend changes the LED state to LED_OFF and resume doesn't change
  38 the state. Please note that there is no explicit interaction between the
  39 suspend and resume actions and the currently enabled trigger. LED state
  40 changes are suspended while the driver is in suspend state. Any timers
  41 that are active at the time driver gets suspended, continue to run, without
  42 being able to actually change the LED state. Once driver is resumed, triggers
  43 start functioning again.
  44
  45 LED state changes are controlled using brightness which is a common led
  46 class device property. When brightness is set to 0 from user space via
  47 echo 0 > brightness, it will result in deactivating the current trigger.
  48
  49 Transient trigger uses standard register and unregister interfaces. During
  50 trigger registration, for each led class device that specifies this trigger
  51 as its default trigger, trigger activation routine will get called. During
  52 registration, the LED state does not change, unless there is another trigger
  53 active, in which case LED state changes to LED_OFF.
  54
  55 During trigger unregistration, LED state gets changed to LED_OFF.
  56
  57 Transient trigger activation routine doesn't change the LED state. It
  58 creates its properties and does its initialization. Transient trigger
  59 deactivation routine, will cancel any timer that is active before it cleans
  60 up and removes the properties it created. It will restore the LED state to
  61 non-transient state. When driver gets suspended, irrespective of the transient
  62 state, the LED state changes to LED_OFF.
  63
  64 Transient trigger can be enabled and disabled from user space on led class
  65 devices, that support this trigger as shown below:
  66
  67 echo transient > trigger
  68 echo none > trigger
  69
  70 NOTE: Add a new property trigger state to control the state.
  71
  72 This trigger exports three properties, activate, state, and duration. When
  73 transient trigger is activated these properties are set to default values.
  74
  75 - duration allows setting timer value in msecs. The initial value is 0.
  76 - activate allows activating and deactivating the timer specified by
  77   duration as needed. The initial and default value is 0.  This will allow
  78   duration to be set after trigger activation.
  79 - state allows user to specify a transient state to be held for the specified
  80   duration.
  81
  82         activate - one shot timer activate mechanism.
  83                 1 when activated, 0 when deactivated.
  84                 default value is zero when transient trigger is enabled,
  85                 to allow duration to be set.
  86
  87                 activate state indicates a timer with a value of specified
  88                 duration running.
  89                 deactivated state indicates that there is no active timer
  90                 running.
  91
  92         duration - one shot timer value. When activate is set, duration value
  93                 is used to start a timer that runs once. This value doesn't
  94                 get changed by the trigger unless user does a set via
  95                 echo new_value > duration
  96
  97         state - transient state to be held. It has two values 0 or 1. 0 maps
  98                 to LED_OFF and 1 maps to LED_FULL. The specified state is
  99                 held for the duration of the one shot timer and then the
 100                 state gets changed to the non-transient state which is the
 101                 inverse of transient state.
 102                 If state = LED_FULL, when the timer runs out the state will
 103                 go back to LED_OFF.
 104                 If state = LED_OFF, when the timer runs out the state will
 105                 go back to LED_FULL.
 106                 Please note that current LED state is not checked prior to
 107                 changing the state to the specified state.
 108                 Driver could map these values to inverted depending on the
 109                 default states it defines for the LED in its brightness_set()
 110                 interface which is called from the led brightness_set()
 111                 interfaces to control the LED state.
 112
 113 When timer expires activate goes back to deactivated state, duration is left
 114 at the set value to be used when activate is set at a future time. This will
 115 allow user app to set the time once and activate it to run it once for the
 116 specified value as needed. When timer expires, state is restored to the
 117 non-transient state which is the inverse of the transient state.
 118
 119         echo 1 > activate - starts timer = duration when duration is not 0.
 120         echo 0 > activate - cancels currently running timer.
 121         echo n > duration - stores timer value to be used upon next
 122                             activate. Currently active timer if
 123                             any, continues to run for the specified time.
 124         echo 0 > duration - stores timer value to be used upon next
 125                             activate. Currently active timer if any,
 126                             continues to run for the specified time.
 127         echo 1 > state    - stores desired transient state LED_FULL to be
 128                             held for the specified duration.
 129         echo 0 > state    - stores desired transient state LED_OFF to be
 130                             held for the specified duration.
 131
 132 What is not supported:
 133 ======================
 134 - Timer activation is one shot and extending and/or shortening the timer
 135   is not supported.
 136
 137 Example use-case 1:
 138         echo transient > trigger
 139         echo n > duration
 140         echo 1 > state
 141 repeat the following step as needed:
 142         echo 1 > activate - start timer = duration to run once
 143         echo 1 > activate - start timer = duration to run once
 144         echo none > trigger
 145
 146 This trigger is intended to be used for for the following example use cases:
 147  - Control of vibrate (phones, tablets etc.) hardware by user space app.
 148  - Use of LED by user space app as activity indicator.
 149  - Use of LED by user space app as a kind of watchdog indicator -- as
 150        long as the app is alive, it can keep the LED illuminated, if it dies
 151        the LED will be extinguished automatically.
 152  - Use by any user space app that needs a transient GPIO output.